The obvious first name to consider is Matthew Broderick, since Brooks has worked with him on "The Producers" (both Broadway and the upcoming movie). And Broderick did the film version of a Don Adams role in "Inspector Gadget." But I'm not sure he would be right for the part. Don Adams' voice and delivery were so distinctively his and so closely associated with our image of Maxwell Smart, it would be hard to pull off an impression.
Never saw the appeal of Matthew Brodrick. One wag shrewdly said that the only emotion he can convey effectively is 'ironic befuddlement'.